Hey Jmor ! I have in fact just for fun cut open several old 12V coils that have said "internal resistor" I have never found a separate resistor.
My shop teacher back in trade school explained it to me this way. The different internal resistance is strictly a function of the size and number of the windings in the coil. The finer the windings the higher the resistance and if you add more windings you get higher resistance. It's pretty basic science if you understand that any conductor has a specific resistance to current flow.